Operators deploy packers and bridge plugs downhole to isolate portions of a borehole for various operations. There are several challenges for such tools. Typically, the packer or bridge plug has a deformable element used to form a seal against the surrounding borehole wall. When being deployed, the deformable element may need to pass through a restriction that is smaller than the diameter of the borehole where the element is to be set. Consequently, the deformed element's size can be limited by the smallest diameter restriction through which it will deploy.
Once deployed at the desired location, the deformable element can then be set by compression, inflation, or swelling depending on the type of element used. Swellable elements take a considerable amount of time (e.g., several days) to swell in the presence of an activating agent, and the swellable elements tend to overly extrude overtime. When an inflatable element is used, it deploys in a collapsed state and then inflates when properly positioned. Unfortunately, the inflatable element can become damaged, can be difficult to implement, and can be affected by changes in downhole temperatures.
In a conventional approach, the packers or plugs use a compression set element having a sleeve that is compressed to increase the element's diameter to form a seal. Compressing such elements can require a great deal of force and a long stroke. To seal against a larger annulus, the sleeve for compressing the element may need to be rather long. Unfortunately, the sleeve may buckle or twist when compressed, leaving unsealed or weak passages on its outer surface where leaking can occur.
Designs for packers and plugs must also deal with extrusion that can occur when packing elements are set. During extrusion, the sealing element's material tends to flow into any gap between the seal bore and a gage ring. If the extrusion is severe, enough of the element's material will no longer be able to maintain a seal with the surrounding borehole wall because it has instead extruded into the gap.
Problems with extrusion also occur with O-rings. Therefore, thermoplastics are often used as back-up rings to stop the extrusion in applications having O-rings. Although the thermoplastic's rigidity helps prevent extrusion, this rigidity makes thermoplastic less useful for packing elements. To create a seal with the wellbore, packing elements must expand outward (circumferentially), and the rigidity of thermoplastics makes them less suited for such an application. Additionally, retrievable packers have to be able to return to a run position to pass through restrictions when running out of hole, which may not be possible with thermoplastics.
One current method of reducing extrusion uses garter springs molded inside the packing elements. These garter springs can expand circumferentially and inhibit extrusion when the packing element is set. Unfortunately, the windings of the springs spread apart from each other when expanded, and this creates gaps through which the packing element's material can extrude.
Another approach to reduce extrusion uses less elastic materials on the ends of the packing elements to contain a more elastic sealing material in the middle of the packing element. The end material needs the elasticity to expand, but also needs the rigidity to resist extrusion. When the extrusion gap is large, finding the right balance between rigidity and elasticity proves difficult.
Some external types of anti-extrusion devices can also be used to prevent extrusion of packing elements. Split rings are one such device that can expand during setting of the packing element and can even engage the surrounding wall of the wellbore or tubular. When the split ring expands, however, the split in the ring creates a large gap through which the element's material can extrude. To overcome this, two split rings are often used with the splits in the rings being offset. Yet, when the packing element's material extrudes into and under these rings, they often must be removed from the well by milling.
Inflatable packers have an inflatable packer element that can be inflated to engage a surrounding sidewall of a tubular. The inflatable element typically has a bladder and outer armor, covers, ribs or the like. During inflation, the inflatable element may develop undesirable folds (commonly referred to as Z-folds) that can compromise any resulting seal. Dealing with the formation of Z-folds has been addressed in the art using techniques such as disclosed in U.S. Pat. Nos. 5,605,195 and 6,752,205.
Shape memory polymers (SMP) are materials known in the art that have shape memory effects. The polymer is processed to receive a permanent shape and is then deformed into a temporary shape using a program process. Typically, this process involves heating up the polymer, deforming it, and then cooling it down, for example. Once programmed, the polymer is fixed in its temporary shape, but the permanent shape is essentially stored. Subsequently heating up the polymer above its transition temperature causes the polymer to revert back to its permanent shape, and cooling down solidifies the material.
Shape memory polymers are different from the types of swelling elastomers used for swellable elements on packers. Swellable elastomers swell in the presence of an activating agent, such as water, hydrocarbon, or other fluid. When the swellable elastomer swells, it absorbs the fluid, changes its volume, and becomes softer as it swells. Shape memory polymers are activated differently by a stimulus that causes the polymer to revert from a temporary shape back to a stored permanent shape of the material. Although the Shape Memory Polymer changes shape, it does not absorb an agent and essentially maintains the same volume.
Shape memory polymers have been described for use in the medical field, for example, in U.S. Pat. No. 6,872,433. These polymers have also been described for use in downhole applications, for example, in U.S. Pat. Nos. 6,896,063 and 7,104,317, as well as in U.S. Pat Pub. Nos. 2005/0202194, 2007/0240877, and 2008/0264647.